The story
Possibly arising from 19th century witticisms unfavorably comparing lawmaking and sausage making, though common usage only appears in the late 1980s and is devoid of references to law.
Etymology from Wiktionary, CC BY-SA 4.0.
how the sausage gets made
Today it means: The unpleasant way in which a process or activity is carried on behind the scenes. Full definition →